1. Field of the Invention
The present invention relates to a novel photosensitive resin composition for making printing plates and to a printing plate precursor comprising this photosensitive resin composition. More specifically, the invention relates to (i) a photosensitive resin composition which is based on a thermoplastic elastomer having poly(amide-imide) segments and can be washed out with water or aqueous solution and (ii) printing plate precursor comprising said photosensitive resin composition, that is, sheets from which printing plates, particularly flexographic printing plates, are made up.
2. Description of the Prior Art
There are various polymers hitherto known as base materials of photosensitive resin compositions for making printing plates. In particular, polymers of the polyamide family have been widely used. Of these polymers, alcohol-soluble copolymer polyamides were chiefly used in the past. These polyamides, however, involve problems in handling or workability and additionally have drawbacks such that fireproof facilities are necessary for the development to make up printing plates. Accordingly, photosensitive resin compositions predominantly used today are of the type which can be washed out with water or aqueous solution.
Polyamides of this type proposed are, for example, those having sulfonic acid groups (Japanese Patent Application Kokai Nos. Sho. 48-68302 and Sho. 48-72250, and so on), those having basic nitrogen or ammonium salt type nitrogen (Japanese Patent Application Kokai Nos. Sho. 50-7605 and Sho. 50-78401, and so on), and those having ether bonds (Japanese Patent Application Kokai No. Sho. 48-42049 and Japanese Patent Publication No. Sho. 49-43565, and so on).
While flexographic printing plates made from various photosensitive resin compositions based on thermoplastic elastomers that have rubber elasticity are known as substitutes for rubber printing plates, most of such substitutes commercialized are of the type made by developing with organic solvents. The use of organic solvents, however, is problematic in public nuisance and the like, prompting investigators to search for a photosensitive elastomeric composition developable with water or aqueous solution. Thus, several compositions (e.g. Japanese Patent Publication Nos. Sho. 59-29849 and Sho. 58-33884) are proposed but none of which are commercialized because of their drawbacks such as low ink resistance.
Other proposed polymers which are superior in water-developability and provide flexible and tenacious printing plates for relief are copolymer polyamides having polyamide main chains into which polyether segments are introduced. These polymers include, for example, copolymer polyamides having constitutional units which consist of (i) polyoxyethylene terminated by amino or carboxy groups and (ii) an aliphatic dicarboxylic acid or diamine (e.g. Japanese Patent Publication Nos. Sho. 57-18173 and Sho. 57-4871) and polyetheresteramides constituted of either a lactam or nylon salt and polyoxyalkylene glycol and dicarboxylic acid (e.g. Japanese Patent Application Kokai Nos. Sho. 58-117537 and Sho. 60-14232). These polyetheresteramides having polyether segments, when forming photosensitive compositions for printing plates, exhibit relatively good characteristics, but their own transparency is insufficient and their compatibility with ethylenic unsaturated compounds used as additives in the compositions is not always high enough. Thus, these resin compositions have drawbacks such that co-usable ethylenic unsaturated compounds are limited in number and the resulting printing plates are unsatisfactory in resolving power.